U.S. patent number 9,631,924 [Application Number 14/890,844] was granted by the patent office on 2017-04-25 for surface profile measurement method and device used therein.
This patent grant is currently assigned to Hitachi High-Technologies Corporation. The grantee listed for this patent is Hitachi High-Technologies Corporation. Invention is credited to Shigeru Matsui, Yugo Onoda.
United States Patent |
9,631,924 |
Matsui , et al. |
April 25, 2017 |
Surface profile measurement method and device used therein
Abstract
To provide a technique that can measure a surface profile of any
test object in a nondestructive manner and noncontact manner,
highly accurately, and in a wide tilt angle dynamic range. In white
light interference method using a dual beam interferometer, the
technique is configured to be capable of changing a surface
orientation of a standard plane with respect to an incident optical
axis on the standard plane, acquires, while relatively changing the
surface orientation of the standard plane with respect to a local
surface orientation in any position on a test surface, a plurality
of interferograms generated by interference of reflected light from
the test surface and reflected light from the standard plane, and
calculates the local surface orientation on the test surface from
the interferograms to thereby measure a surface profile of the test
surface.
Inventors: |
Matsui; Shigeru (Tokyo,
JP), Onoda; Yugo (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi High-Technologies Corporation |
Minato-ku, Tokyo |
N/A |
JP |
|
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Assignee: |
Hitachi High-Technologies
Corporation (Tokyo, JP)
|
Family
ID: |
51898119 |
Appl.
No.: |
14/890,844 |
Filed: |
March 10, 2014 |
PCT
Filed: |
March 10, 2014 |
PCT No.: |
PCT/JP2014/056092 |
371(c)(1),(2),(4) Date: |
November 12, 2015 |
PCT
Pub. No.: |
WO2014/185133 |
PCT
Pub. Date: |
November 20, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20160091304 A1 |
Mar 31, 2016 |
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Foreign Application Priority Data
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|
|
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May 14, 2013 [JP] |
|
|
2013-101792 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01B
9/02068 (20130101); G01B 9/0209 (20130101); G01B
9/04 (20130101); G01B 11/2441 (20130101); G01B
11/26 (20130101); G01B 11/0608 (20130101); G01B
9/02087 (20130101) |
Current International
Class: |
G01B
11/24 (20060101); G01B 11/06 (20060101); G01B
11/26 (20060101); G01B 9/02 (20060101); G01B
9/04 (20060101) |
Field of
Search: |
;356/511-514 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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5-240628 |
|
Sep 1993 |
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JP |
|
3295846 |
|
Jun 2002 |
|
JP |
|
2002-202112 |
|
Jul 2002 |
|
JP |
|
2006-242853 |
|
Sep 2006 |
|
JP |
|
2006-258557 |
|
Sep 2006 |
|
JP |
|
Other References
International Search Report (PCT/ISA/210) issued in counterpart
International Application No. PCT/JP2014/056092 dated Apr. 22,
2014, with English translation (Three (3) pages). cited by
applicant .
Sato, A., "Advanced Metrology of Surface Texture by Scanning White
Light Interferometry", The Journal of the Surface Finishing Society
of Japan, vol. 57, No. 8, 2006, pp. 554-558, with partial English
translation (Nine (9) pages). cited by applicant .
Kondo, Y., "A Survey on Surface Metrology for Flatness Standard",
AIST Bulletin of Metrology, vol. 8, No. 3, Sep. 2011, pp. 299-310
with partial English translation (Sixteen (16) pages). cited by
applicant .
Extended European Search Report issued in counterpart European
Application No. 14798582.4 dated Dec. 8, 2016 (11 pages). cited by
applicant.
|
Primary Examiner: Hansen; Jonathan
Attorney, Agent or Firm: Crowell & Moring LLP
Claims
The invention claimed is:
1. A surface profile measurement method for comparing a test
surface and a standard plane to thereby measure both of a surface
height and a surface orientation of the test surface in any
position on the test surface, the surface profile measurement
method being configured to capable of determining, only with
measurement data in one position on the test surface, both of the
surface height and the surface orientation without requiring to
calculate the surface height and the surface orientation from
measurement data in two or more positions on the test surface and
by changing the standard plane compared with the test surface.
2. A surface profile measurement method for dividing an
illumination light beam emitted from a light source continuously or
discretely having a predetermined wavelength bandwidth or a light
source for emitting monochromatic light into two light beams and
making the light beams incident on a test surface and a standard
plane, and causing a reflected light beam from the test surface and
a reflected light beam from the standard plane to interfere in an
interferometer to measure a surface profile of the test surface,
the surface profile measurement method being configured to be
capable of changing a surface orientation in the illumination light
beam incident position on the standard plane and configured to be
capable of measuring a local surface orientation of the test
surface in one or a plurality of positions on the test surface.
3. The surface profile measurement method according to claim 2,
wherein the surface orientation in the illumination light beam
incident position on the standard plane can be changed by
configuring the surface profile measurement method to be capable
of, using a plane mirror as the standard plane, inclining or
rotating the surface orientation of the standard plane in two axial
directions orthogonal to each other and both orthogonal to an
optical axis.
4. The surface profile measurement method according to claim 2,
wherein the surface orientation in the illumination light beam
incident position on the standard plane can be changed by using, as
the standard plane, a curved surface mirror, a local surface
orientation of a reflective surface of which continuously or
discretely changes in two axial directions orthogonal to each other
and both orthogonal to an optical axis, and translating the entire
standard plane in the two axial directions orthogonal to the
optical axis.
5. The surface profile measurement method according to claim 2,
wherein the surface profile measurement method is configured to
measure a local surface orientation of the test surface by changing
the surface orientation of the standard plane relatively to a local
surface orientation in any position on the test surface and
calculating a surface orientation of the standard plane at time
when the local surface orientation on the test surface and a
surface orientation in the illumination light beam incident
position on the standard plane are equal.
6. The surface profile measurement method according to claim 5,
wherein the surface profile measurement method is configured to
determine that, when an interference contrast is maximized in an
interferogram obtained by interference of a reflected light beam
from the test surface and a reflected light beam from the standard
plane, the local surface orientation on the test surface and the
surface orientation of the standard plane are equal.
7. The surface profile measurement method according to claim 6,
wherein the surface profile measurement method is configured to
calculate a relative standard deviation from interference intensity
data configuring the interferogram and use the relative standard
deviation as the interference contrast.
8. The surface profile measurement method according to claim 6,
wherein the surface profile measurement method is configured to, in
calculating a surface orientation of the standard plane at the time
when the interference contrast is maximized, instead of finding a
maximum value out of interference contrasts, which are measured by
changing the surface orientation continuously or discretely at a
sufficiently fine pitch, and calculating the surface orientation
corresponding to the maximum value, match a predetermined
interference contrast distribution function with a plurality of
interference contrast values, which are measured by roughly
discretely changing the surface orientation, using a surface
orientation corresponding to the maximum value of the interference
contrast as an unknown number and calculate the surface orientation
as a surface orientation at time when the interference contrast
distribution function matches the interference contrast value
most.
9. The surface profile measurement method according to claim 2,
wherein, when a Z axis is plotted in an illumination optical axis
direction with respect to the test surface and a value of a Z
coordinate of the test surface is referred to as height of the test
surface, the surface profile measurement method includes a function
of obtaining, from surface orientation data measured in two or more
positions on the test surface, surface height calculation values of
the test surface in the positions according to integration
processing.
10. The surface profile measurement method according to claim 2,
wherein the surface profile measurement method is configured to
measure both of a local surface orientation and height of the test
surface using an interferogram obtained in any position on the test
surface.
11. The surface profile measurement method according to claim 10,
wherein the surface profile measurement method is configured to
irradiate, as the illumination light beam, illumination light
continuously or discretely having a predetermined wavelength
bandwidth on the test surface and the standard plane.
12. The surface profile measurement method according to claim 10,
wherein the surface profile measurement method is configured to
include, as the illumination light beam, a light source
continuously or discretely having a predetermined wavelength
bandwidth for measuring the height and a monochromatic light source
for measuring the local surface orientation and simultaneously or
sequentially irradiate illumination lights from the light sources
on the test surface and the standard plane.
13. The surface profile measurement method according to claim 1,
wherein the surface profile measurement method includes a sample
moving stage for moving a position of a sample having the test
surface in order to change a measurement position on the test
surface, includes a function of obtaining, from surface orientation
data measured in two or more positions on the test surface, surface
height calculation values in the positions according to integration
processing, and includes a function of comparing surface heights
measured in the positions and the surface height calculation
values.
14. The surface profile measurement method according to claim 13,
wherein the surface profile measurement method is configured to be
capable of obtaining, on the basis of a result obtained by
comparing the surface heights measured in the positions and the
surface height calculation values, a height fluctuation
distribution of the entire sample involved in movement of the
sample moving stage.
15. A surface profile measurement device comprising: a light source
continuously or discretely having a predetermined wavelength
bandwidth; an interferometer including a sample moving stage
configured to divide, with a beam splitter, an illumination light
beam into a measurement light beam and a reference light beam,
reciprocatingly move the measurement light beam between a sample
and a test surface through a first objective lens, and
reciprocatingly move the reference light beam between the sample
and a standard plane through a second objective lens manufactured
to have a characteristic same as a characteristic of the first
objective lens, the sample moving stage being capable of mounting
the sample including the test surface and moving a measurement
position and changing a relative distance in a direction along an
optical axis of the measurement light beam between the sample and
the first objective lens, after giving a change to an optical path
difference represented by a relative difference between an optical
length for reciprocatingly moving the measurement light beam
between the sample and the measurement surface and an optical
length for reciprocatingly moving the reference light beam between
the sample and the standard plane, the interferometer recombining
the measurement light beam and the reference light beam after the
reciprocating movement and causing the light beams to interfere; a
control unit that controls the sample moving stage and the
interferometer; a photodetector that converts intensity of
interference light obtained by the interferometer into an electric
signal; and a data processing unit that processes an output signal
of the photodetector, the surface profile measurement device
controlling the sample stage and the interferometer in a plurality
of sampling positions on the test surface and subjecting an output
signal output from the photodetector to calculation processing in
the data processing unit to thereby measure a surface profile of
the test surface, wherein a plane mirror is used as the standard
plane of the interferometer, a surface orientation of the standard
plane is set to be capable of inclining or rotating in two axial
directions orthogonal to each other or both orthogonal to an
optical axis, the control unit is configured to change the surface
orientation of the standard plane in the two axial directions and
change a position of the sample moving stage in a direction along
an optical axis of the measurement light beam to thereby change the
optical path difference, the photodetector is configured to convert
an intensity change of the interference light involved in a change
in the surface orientation and the optical path difference into an
electric signal, the data processing unit is configured to subject
a change in the electric signal to calculation processing to detect
local surface orientations of the test surface in the sampling
positions on the test surface.
16. The surface profile measurement device according to claim 15,
wherein, in the detection of the local surface orientation, the
surface profile measurement device is configured to change the
surface orientation of the standard plane to a plurality of
orientations stepwise in the two axial directions, match a
predetermined interference contrast distribution function with an
interference contrast of the interference light observed when the
optical path difference is scanned in a predetermined range using,
as an unknown number, a surface orientation corresponding to a
maximum value of the interference contrast, and calculate the
surface orientation as a surface orientation at time when the
interference contrast distribution function matches the
interference contrast value most.
17. The surface profile measurement device according to claim 15,
wherein, when a Z axis is plotted in an illumination optical axis
direction with respect to the test surface and a value of a Z
coordinate of the test surface is referred to as height of the test
surface, the surface profile measurement device includes a function
of obtaining, from surface orientation data measured in two or more
positions on the test surface, surface height calculation values of
the test surface in the positions according to integration
processing.
18. The surface profile measurement device according to claim 15,
wherein the surface profile measurement device is configured to
measure both of a local surface orientation and height of the test
surface using an interferogram obtained in any position on the test
surface.
Description
TECHNICAL FIELD
The present invention relates to a surface profile measurement
method and a surface profile measurement device for measuring a
surface profile of a three-dimensional object. For example, in
particular, the present invention relates to a surface profile
measurement method and a surface profile measurement device
suitable for measuring an optical element, a reflective surface or
a refractive surface of which consists of a curved surface, in a
nondestructive manner and noncontact manner, highly accurately, and
in a wide tilt angle dynamic range using light.
BACKGROUND ART
As a technique for measuring a surface profile of a
three-dimensional object in a nondestructive and noncontact manner
and highly accurately using light, for example, as described in NPL
1 and U.S. Pat. No. 5,398,113 specification (publication) (PTL 1),
there has been a technique for combining a light source, which
emits white light, and a dual beam interferometer and detecting,
with a two-dimensional image sensor, an interference figure (an
interferogram) obtained by causing reflected light from a micro
region on a sample surface and reflected light from a standard
plane incorporated in the dual beam interferometer to interfere
with each other through an objective lens to thereby measure a
height distribution of the sample surface. In this technique, in
each of pixels of the two-dimensional image sensor, the reflected
light from the sample surface made incident to an effective light
sensing area of the pixel and the reflected light from the standard
plane cause interference. At least during surface profile
measurement of the sample, a surface orientation of the standard
plane is fixed and used without being configured to be changed with
respect to an incident optical axis of the reflected light.
Information concerning a tilt angle distribution of the sample
surface is not directly measured. JP-A-2006-242853 (PTL 2)
discloses a technique including a mechanism for, instead of setting
a standard plane having high surface accuracy as a standard plane,
setting, in a dual beam interferometer used in monochromatic
interferometry, a reference object having a surface profile
substantially equal to a surface profile of a sample and adjusting
a surface orientation of the standard plane.
On the other hand, as another conventional technique, for example,
as described in pp. 306 to 307 of NPL 2, there is also a technique
for measuring a tilt angle distribution on a sample surface using
an autocollimator. In this technique, it is also possible to obtain
a height distribution on the sample surface by integrating the tilt
angle distribution.
CITATION LIST
Patent Literature
PTL 1: U.S. Pat. No. 5,398,113 PTL 2: JP-A-2006-242853
Non Patent Literature
NPL 1: "Advanced Metrology of Surface Texture by Scanning White
Light Interferometry", Atsushi SATO, The journal of the Surface
Finishing Society of Japan, Vol. 57. No. 8, pp. 554 to 558, issued
in 2006 NPL 2: "A survey on surface metrology for flatness
standard", Yohan KONDO, AIST bulletin of Metrology, Vol. 8, No. 3,
pp. 299 to 310, issued in September 2011
SUMMARY OF INVENTION
Technical Problem
In the surface profile measurement technique of the white light
interference system described in U.S. Pat. No. 5,398,113
specification (publication) (PTL 1), wave fronts of the two
reflected lights are parallel. That is, when an angle formed with
respect to a surface orientation in a measured region corresponding
to the pixel on the sample surface and an incident optical axis on
the measured region and an angle formed by a surface orientation of
the standard plane and an incident optical axis on the standard
plane are the same, since an optical path difference between the
two reflected lights is equal irrespective of a place in the pixel,
a uniform interference effect is obtained. However, when the two
wave fronts are not parallel and tilt at a certain angle each
other, since the optical path difference changes between the two
reflected lights according to a place in the pixel, the
interference effect is not uniform. When a difference between
optical path differences in the pixel is equal to or larger than an
illumination wavelength, since the interference effect is cancelled
by averaging, a surface profile cannot be measured. Further, to
enable detection at a sufficient S/N without attenuating the
interference effect, the difference between the optical path
differences in the pixel needs to be kept within approximately a
half of the illumination wavelength. In the technique, at least
during surface profile measurement of the sample, the surface
orientation of the standard plane is fixed and used without being
configured to change with respect to the incident optical axis of
the reflected light. Therefore, when the surface orientation in the
measured region on the sample surface changes, a situation in which
the interference effect is attenuated occurs in this way.
The width of each of the pixels is represented as d, a point image
width of the objective lens is represented as d', the illumination
wavelength is represented as .lamda., and a difference between the
angle formed with respect to the surface orientation in the
measured region corresponding to the pixel on the sample surface
and the incident optical axis on the measured region and the angle
formed by the surface orientation of the standard plane and the
incident optical axis on the standard plane is represented as
.theta.. The point image width d' indicates width from a foot on
one side where the intensity of a point spread function of the
objective lens is sufficiently small to a foot on the other side.
In this case, d' is approximately 1.6 times as large as a Rayleigh
limit often used in general as a resolution limit. If d is larger
than d', when dtan 2.theta..gtoreq..lamda./2 Expression 1,
attenuation of the interference effect occurs. If d is smaller than
d', replacing d of Expression 1 with d', when d'tan
2.theta..gtoreq..lamda./2 Expression 2, attenuation of the
interference effect occurs. In both the cases, to prevent the
interference effect from being attenuated, the expression has to be
dtan 2.theta..gtoreq..lamda./2 Expression 3. When .theta. exceeds a
range in which Expression 3 is satisfied, surface profile
measurement is difficult. When visible light is used as the
illumination light, the center wavelength of the visible light is
approximately .lamda.=600 nm. In an objective lens having a large
working distance suitable for the surface profile measurement,
since a numerical aperture (NA) is as large as approximately
NA=0.55, d' is equal to or larger than approximately 1.06
micrometers. At this point, when the inclination angle of the
sample surface increases and .theta..gtoreq.7.9.degree., Expression
3 is not satisfied. The surface profile measurement making use of
the interference effect is difficult.
On the other hand, the technique disclosed in JP-A-2006-242853
(Patent Literature 2) includes a mechanism for adjusting the
surface orientation of the standard plane. It is taken into account
that the interference effect in a place with a large inclination
angle on the sample surface is secured. However, the mechanism is
used to optimize, on the entire sample surface, alignment between
an optical axis in the dual beam interferometer and optical
elements before height distribution measurement of the sample is
started. The technique is based on the premise that the sample
surface and the standard plane have substantially equal surface
profile distributions. Therefore, a situation in which the surface
geometries of the sample surface and the standard plane are locally
different is not taken into account. The alignment is only
performed for the entire sample surface. Therefore, in the
technique, the height distribution itself of the sample surface
cannot be directly obtained. Only a distribution of a deviation of
the height of the sample surface with respect to a height
distribution of the reference object surface set as the standard
plane can be measured. A technique for measuring information
concerning the tilt angle distribution of the sample surface is not
included either. In this way, in the technique, a surface profile
of a sample having any surface profile cannot be measured.
On the other hand, in the surface profile measurement technique for
measuring a tilt angle distribution of a sample surface using the
autocollimator described in pp. 306 to 307 of NPL 2, a measurement
range of a high-precision autocollimator is approximately .+-.
several ten seconds to .+-. several hundred seconds. A surface
profile set as a measurement target is limited to a plane or a
gentle curved surface. When the inclination angle of the sample
surface increases, surface profile measurement is difficult.
The present invention has been devised in view of the above and it
is an object of the present invention to provide a technique that
can measure a surface profile of any test object in a
nondestructive manner and noncontact manner, highly accurately, and
in a wide tilt angle dynamic range.
Solution to Problem
In order to attain the object, the present invention provides, in
white light interference method using a dual beam interferometer, a
technique for configuring a surface orientation of a standard plane
to be changed with respect to an incident optical axis on the
standard plane, acquiring, while relatively changing the surface
orientation of the standard plane with respect to a local surface
orientation in any position on a test surface, a plurality of
interferograms generated by interference of reflected light from
the test surface and reflected light from the standard plane, and
calculating the local surface orientation on the test surface from
the interferograms to thereby measure a surface profile of the test
surface.
Advantageous Effect of Invention
In the present invention, it is possible to not only measure a
surface profile of any test object in a nondestructive and
noncontact manner using light but also measure the surface profile
highly accurately and in a wide tilt angle dynamic range.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing the configuration of a surface profile
measurement device according to a first embodiment of the present
invention.
FIG. 2(a) FIG. 2A is a diagram showing an example of an
interferogram according to the first embodiment of the present
invention (in the case of a monochromatic light source).
FIG. 2(b) FIG. 2B is a diagram showing an example of an
interferogram according to the first embodiment of the present
invention (in the case of a broad spectral band light source).
FIG. 3 is a diagram showing an operation flow of the surface
profile measurement device according to the first embodiment of the
present invention.
FIG. 4 is a diagram showing an effect at the time when a surface
orientation of a standard plane is changed by the surface profile
measurement device according to the first embodiment of the present
invention.
FIG. 5 is a diagram showing the configuration of a surface profile
measurement device according to a second embodiment of the present
invention.
DESCRIPTION OF EMBODIMENTS
Embodiments of the present invention are explained below with
reference to the drawings.
First Embodiment
FIG. 1 is a diagram showing the configuration of a surface profile
measurement device according to a first embodiment of the present
invention. As a light source 1, a broad spectral band light source
that generates light having a continuous wavelength such as a
halogen lamp, a Xe lamp, or an LED is used. A light beam emitted
from the light source 1 changes to a parallel beam 30 through an
illumination optical system 2 including a lens or a reflection
mirror for condensing and light beam parallelization and is made
incident on a beam splitter 4 in a dual beam interferometer 3. The
parallel beam 30 is divided into two light beams by the beam
splitter 4. One divided light beam is reflected after being made
incident on an illumination region 41 on the surface of a sample 40
through a sample-side objective lens 5, changes to a sample-side
reflected light beam 31, and returns to the beam splitter 4 through
the sample-side objective lens 5 again. The sample 40 is mounted on
a sample moving stage 11 and is movable in orthogonal two axial
directions (an X axis and a Y axis) and an optical axis direction
(a Z axis) with respect to an optical axis of the sample-side
objective lens 5. The X axis and the Y axis are used to move the
position of the illumination region 41 on the sample 40. An
X-coordinate value and a Y-coordinate value are controlled by an
X-Y driving/control unit 12. The Z axis is driven by a piezo
actuator 13 (not shown in the figure). A Z-coordinate value can be
controlled by a Z-axis control unit 14 at resolution of
approximately 1 nanometer. The other of the two light beams divided
by the beam splitter 4 is made incident on a standard plane 7
through a reference-side objective lens 6 and thereafter changes to
a reference-side reflected light beam 32 and returns to the beam
splitter 4 through the reference-side objective lens 6 again. The
sample-side objective lens 5 and the reference-side objective lens
6 are set such that distances from the beam splitter 4 are equal to
each other. The standard plane 7 is set in a focusing position of
the reference-side objective lens 6. An inclination angle of the
standard plane 7 can be changed with respect to two axes orthogonal
to an optical axis and corresponding to X and Y axes of the sample
moving stage 11 by a two-axis inclining mechanism 15. In the
following explanation, an inclination angle in a direction
corresponding to the X axis is represented as .theta.x and an
inclination angle in a direction corresponding to the Y axis is
represented as .theta.y. .theta.x and .theta.y are respectively
driven by piezo actuators 16 and 17 (not shown in the figure).
Angle control can be performed by an inclination-angle control unit
18 at resolution of approximately 5 micro-radians. The sample-side
reflected light beam 31 and the reference-side reflected light beam
32 returning to the beam splitter 4 in this way are wave-optically
combined to generate an interference light beam 33. After the
interference light beam 33 is made incident on a focusing lens 8, a
part of the interference light beam 33 passes through a field stop
9 set on a focusing surface of the focusing lens. The focusing lens
8 is adjusted to focus an image of the illumination region 41 on
the focusing surface in a state in which the illumination region 41
is placed in a focusing position of the sample-side objective lens
5. The interference light beam 33 passed through the field stop 9
is lead to a photodetector 10. The light intensity of the
interference light beam 33 is converted into an electric signal.
The interference light beam 33 changes to an interference light
intensity signal 34. The interference light intensity signal 34 is
captured into the computer 21 through an A/D converter 20 and
subjected to arithmetic processing. The computer 21 gives commands
to the X-Y driving/control unit 12, the Z-axis control unit 14, and
the inclination-angle control unit 18 and causes the units to
change the X-coordinate value, the Y-coordinate value, the
Z-coordinate value, and values of .theta.x and .theta.y.
In general, the dual beam interferometer represented by a Michelson
interferometer artificially gives a change in a phase difference to
between divided light beams and thereafter recombines the light
beams, causes the light beams to interfere, and records a change in
interference light intensity involved in the change in the phase
difference. Numerical value data of the change in the interference
light intensity involved in the change in the phase difference, a
figure obtained by graphing the numerical value data, or an optical
image obtained by spatially generating the change in the
interference light intensity as a light amount distribution of
light and shade is called interferogram (interference figure). The
phase difference depends on an optical path difference between
optical paths of tracing of the two light beams from the division
to the recombination, that is, a difference between optical lengths
and the wavelength of light in use. In the optical system in this
embodiment, the optical path difference between the two light beams
is a difference of an optical path of the beam splitter
4.fwdarw.the sample-side objective lens 5.fwdarw.the illumination
region 41 on the sample 40.fwdarw.the sample-side objective lens
5.fwdarw.the beam splitter 4.fwdarw.and an optical length of an
optical path of the beam splitter 4.fwdarw.the reference-side
objective lens 6.fwdarw.the standard plane 7.fwdarw.the
reference-side objective lens 6.fwdarw.the beam splitter 4. When
the phase difference is represented as .phi. radians, the optical
path difference between the two divided light beams is represented
as .DELTA.L micrometers, and a wavelength in use is represented as
.lamda. micrometers, .phi.=2.pi..DELTA.L/.lamda. Expression 4 is
obtained. Therefore, the dual beam interferometer is often
configured to place a reflection mirror in the optical path of one
of the two light beams and translate the position of the reflection
mirror to thereby change the optical length and record an
interferogram. When a light source in use is a monochromatic light
source that emits only light having a single wavelength, an equal
interference light intensity change repeatedly occurs every time
the optical path difference becomes twice as large as the
wavelength of the light source. Therefore, an interferogram
consisting of a single COS waveform shown in FIG. 2a is obtained.
On the other hand, when a broad spectral band light source that
generates light having a continuous wavelength is used,
interference between two light beams is so-called white light
interference. It is well known that, as shown in FIG. 2b, light
intensity takes a maximum value in an optical path difference (a
zero optical path difference) at which phase differences are
substantially zero in common at wavelengths included in the light
source and a vibration waveform is observed only around the optical
path difference.
The operation of the computer 21 after the sample 40 is mounted on
the sample moving stage 11 is explained using an operation flow in
FIG. 3. Processing of the operation flow in FIG. 3 is incorporated
in the computer 21 as an inclination-angle measuring function
50.
In this embodiment, as shown in Step 9 to Step 12, the computer 21
gives a command to the Z-axis control unit 14 and causes the Z-axis
control unit 14 to move the Z-coordinate value from a predetermined
initial position to an end position and captures the interference
light intensity signal 34 to thereby record one interferogram. The
initial position and the end position are determined to include the
zero optical path difference. The shape of the interferogram
obtained at this point is generally as shown in FIG. 2b. An amount
serving as an interference contrast C is defined. When a light
source in use is a monochromatic light source, the interference
contrast C is generally defined by the following expression. In the
expression, the Z coordinate is changed to Z0, Z1, . . . , and Zn
at a fixed interval, interference light intensity in Zi is
represented as Ji, max{Ji} represents a maximum value among J0, J1,
. . . , and Jn, and min{Ji} represents a minimum value.
C=[max{Ji}-min{Ji}]/[max{Ji}+min{Ji}] Expression 5 However, when a
white light source is used, since the vibration waveform of the
interference intensity is observed only around the zero optical
path difference as shown in FIG. 2b and an envelope of vibration is
attenuated as the optical path difference is further away from the
zero optical path difference, the definition by the above
expression is inappropriate. Therefore, in the present invention,
the interference contrast C is defined by the following expression
when a z coordinate of a zero optical path difference position is
represented as Zc, the vibration waveform of the interference
intensity is observed in a range of Za to Zb, and an average of the
interference intensity {Ji} in a range of a.ltoreq.i.ltoreq.b is
represented as J0. b C=[{.SIGMA.(Ji-J0)^2}/(b-a+1)}]^(1/2)/J0 i=a
Expression 6
Expression 6 is equal to a relative standard deviation of {Ji} in
the range of a.ltoreq.i.ltoreq.b. The calculation of the
interference contrast is performed in Step 13. In this embodiment,
as shown in Step 5 to Step 15, the recording of one interferogram
is performed every time the computer 21 gives a command to the
inclination-angle control unit 18 and causes the inclination-angle
control unit 18 to move .theta.x and .theta.y from predetermined
initial positions to end positions by a predetermined pitch.
A result of an actually performed test using a dual beam
interferometer same as the configuration in this embodiment is
shown in FIG. 4. In the test, as the sample 40, a plane mirror 42,
to an optical axis of which a predetermined inclination angle was
given in advance, was placed, an inclination angle of the standard
plane 7 was changed, and a relation between the inclination angle
of the standard plane 7 obtained at that point and the interference
contrast C was checked. As a result of the test, it was confirmed
that the interference contrast C was maximized when the inclination
angle of the plane mirror 42 placed as the sample and the
inclination angle of the standard plane 7 were equal to each other.
In this test, as differences from the configuration in this
embodiment, the field stop 9 was detachably attachable, a CCD
camera was able to be placed instead of the field stop 9, and an
image formed when the interference light beam 33 was focused by the
focusing lens 8 was able to be observed. As a result, it was found
that, when the inclination angles of the plane mirror 42 and the
standard plane 7 were different, interference fringes of light and
shade appeared on an image acquired by the CCD camera, an interval
of the interference fringes increased as the difference between the
inclination angles decreased, and, when the inclination angles were
equal and had no difference, the interference fringes were not
observed. From the two test results, it is seen that, when the
inclination angles of the plane mirror 42 and the standard plane 7
are equal, the optical path difference is equal in the entire
region of the field stop 9 and the interference fringes of light
and shade are not observed and, when the plane mirror 42 is moved
in the optical axis direction, since all phases of light beams
passing in the region uniformly change, the interference contrast C
is maximized. In general, a surface profile of the sample 40 needs
to be considered a non-plane. However, the surface profile in the
illumination region 41 in a microscopic sense can be approximately
regarded as being sufficiently a plane under an optical microscope.
Therefore, from the test results, it is seen that an inclination
angle of a local micro plane in the illumination region 41 on the
sample 40 can be measured by detecting an inclination angle of the
standard plane 7 at the time when the interference contrast Cis
maximized. Even when the inclination angle of the plane mirror 42
increases, by also increasing the inclination angle of the standard
plane 7 according to the increase in the inclination angle, the
phases of the light beams passing in the region can be uniformly
aligned and the interference contrast can be secured.
Referring back to the operation flow in FIG. 3, in this embodiment,
in Step 16, a set of (.theta.x, .theta.y) for maximizing the
interference contrast C is detected. In order to obtain a set of
(.theta.x, .theta.y) serving as a solution at high accuracy, in
Step 14 and Step 15, it is necessary to set the pitch in moving
.theta.x and .theta.y sufficiently fine. However, as shown in FIG.
4, the interference contrast C shows only an extremely gentle
change with respect to a change in .theta.x and .theta.y near a
maximum point of the interference contrast C. When a measurement
result of the interference contrast C wavers because of
superimposition of noise, a large error is caused. Therefore, in
the present invention, in Step 16, a predetermined fitting function
F(.theta.x, .theta.y) is fit to numerical values of a plurality of
interference contrasts C obtained in Step 5 to Step 15 by a method
of least squares using a value of (.theta.x, .theta.y)
corresponding to a vertex position as an unknown number. A set of
(.theta.x, .theta.y) obtained as a most matching result is adopted
as a solution. The set of (.theta.x, .theta.y) obtained at this
point is measurement values of inclination angles in two axial
directions of X-Y on the local micro plane in the illumination
region 41 on the sample 40 mounted on the sample moving stage 11.
In order to obtain inclination angles for all surfaces on the
sample 40, as shown in Step 1 to Step 18, the sample moving stage
11 is moved in the X-Y directions and the processing shown in Step
5 to Step 16 is repeated. In this way, in this embodiment, a tilt
angle distribution (.theta.x, .theta.y) can be measured on the all
the surfaces on the sample 40.
The inclination angles (.theta.x, .theta.y) in the two axial
directions of X-Y on the local micro plane in the illumination
region 41 on the sample 40 mounted on the sample moving stage 11
are differential values of a sample surface Z=F(X, Y) in the local
plane position. That is,
.theta.x=.differential.F(X,Y)/.differential.X,
.theta.y=.differential.F(X,Y)/.differential.Y Expression 7
Therefore, by integrating (.theta.x, .theta.y) on a two-dimensional
plane of X-Y by giving an appropriate initial value, conversely, it
is possible to reconstruct a distribution of Z=F (X, Y). In this
embodiment, the computer 21 also includes an inclination
angle/height converting function 51 for converting an inclination
angle into Z height according to this integration conversion. It is
possible to calculate a height distribution Z=F (X, Y) from the
distribution of the inclination angles (.theta.x, .theta.y)
measured as explained above.
In this way, in this embodiment, a height distribution and a tilt
angle distribution can be measured as a surface profile of any test
object in a nondestructive manner and noncontact manner, highly
accurately, and in a wide tilt angle dynamic range using light.
Second Embodiment
A second embodiment of the present invention is explained with
reference to FIG. 5, which is a configuration diagram in the second
embodiment.
In this embodiment, a mechanism for measuring the height Z of the
local micro plane in the illumination region 41 on the sample 40 is
added to the first embodiment to make it possible to evaluate an
up-down fluctuation characteristic of a sample moving stage. As
explained above, in the first embodiment, it is possible to
calculate the height distribution Z=F (X, Y) by directly measuring
the distribution of the inclination angles (.theta.x, .theta.y).
However, in addition to this, this embodiment has a function of
directly measuring the height distribution Z=F (X, Y) using a dual
beam interferometer. An optically directly measured height
distribution is represented as Z1=F1 (X, Y) and a height
distribution calculated by integrating a tilt angle distribution is
represented as Z2=F2(X, Y) to distinguish the height distributions.
In Z1, not only height information of the sample 40 but also
undesired up-down height fluctuation of the stage surface in
driving the sample moving stage 11 to move the measurement position
is included as an error. On the other hand, when the stage surface
moves up and down according to the driving, if fluctuation in an
angle direction is sufficiently small, since inclination angle
measurement is hardly affected by the fluctuation, Z2 does not
involve an error. Therefore, it is possible to evaluate a height
fluctuation characteristic of the sample moving stage 11 by
calculating a difference of Z1-Z2. Therefore, in this embodiment,
in the computer 21, a height measuring function 52 and a
height-difference detecting function 53 are provided in addition to
the inclination-angle measuring function 50 and the inclination
angle/height converting function 5l. The other components are the
same as the components in the first embodiment.
In this embodiment configured as explained above, besides the
effects obtained in the first embodiment, it is possible to
evaluate the up-down fluctuation characteristic of the sample
moving stage.
REFERENCE SIGNS LIST
1 Light source 2 Illumination optical system 3 Dual beam
interferometer 4 Beam splitter 5 Sample-side objective lens 6
Reference-side objective lens 7 Standard plane 8 Focusing lens 9
Field stop 10 Photodetector 11 Sample moving stage 12 X-Y
driving/control unit 13, 16, 17 Piezo actuators 14 Z-axis control
unit 15 Two-axis inclining mechanism 18 Inclination-angle control
unit 20 A/D converter 21 Computer 30 Parallel beam 31 Sample-side
reflected light beam 32 Reference-side reflected light beam 33
Interference light beam 34 Interference light intensity signal 35
Interferogram 40 Sample 41 Illumination region 50 Inclination-angle
measuring function 51 Inclination angle/height converting function
52 Height measuring function 53 Height-difference detecting
function
* * * * *